Ascorbate monitoring and control system

ABSTRACT

Method and system embodiments of the present invention control the ascorbate concentration in produce treatments and particularly are exemplified in fresh cut fruit and vegetable treatments via measured refractivity and/or electrical conductivity of, and/or calcium ions present in, the treatment solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/016,610 filed Jan. 28, 2011 now U.S. Pat. No. 8,323,571, which is adivisional of U.S. Pat. No. 7,998,513, issued Aug. 16, 2011, whichclaims the benefit of U.S. Provisional Patent Application No. 60/948,900filed Jul. 10, 2007, the contents of which, including all appendices,are hereby incorporated by reference herein for all purposes.

BACKGROUND

1. Field of Endeavor

The invention, in its several embodiments, pertains generally to themeasurement and control of ascorbate in liquid solutions andparticularly to the monitoring and control of ascorbate based onrefractivity, and/or the electrical conductivity, of a liquid solutionsample and/or the monitoring and control of calcium ascorbate based onthe concentration of calcium ions in the liquid solution.

2. State of Technology

Ascorbate is a salt, e.g., calcium ascorbate and sodium ascorbate, orother derivatives of ascorbic acid. Ascorbate compound solution is usedas an anti-oxidant dip for the prevention of the browning of fruit andvegetable surfaces, and preserving the appearance, texture, crispnessand color of fresh cut or minimally processed fruits and vegetables,such as apples. Proper management via continuous or continual monitoringand control of ascorbate concentration levels in the dip tank duringprocessing enhances the effectiveness of the ascorbate as a preservativefor fresh fruit and vegetables. Brine solutions containing ascorbic acidhave been proposed in U.S. Pat. No. 4,883,679 to Sewón. Compositions ofedible ingredients in solution and a dipping process for treatingfreshly cut surfaces of edible plants have been proposed in U.S. Pat.No. 4,988,522 to Warren. Methods of preserving fresh fruit have beenproposed such as: (a) using calcium ascorbate according to U.S. Pat. No.5,939,117 to Chen et al.; (b) using ascorbic acid according to U.S. Pat.No. 6,749,875 to Selleck; and (c) using calcium ions and ascorbate ionsaccording to WO 94/12041 by Lidden.

Calcium ion concentrations in solutions can be measured based on theelectrical properties of the solution, particularly by electrodes thatare ion specific. Calcium ion-specific electrodes have been proposed foruse in the measuring of calcium ions, e.g., as disclosed by U.S. Pat.No. 4,724,216 to Young et al., and by EP0304151 to Musacchio et al.

A refractometer is an instrument that measures the refractive indexbased on a principle that the refractive index of a solution increasesin proportion to the concentration of the solute in the solution. Asillustrated in FIG. 1, if the refractive index of air under atmosphericpressure is unity, when light enters medium x, the ratio of the sine ofthe incident angle, α, measured against the phase boundary to the sineof the refracting angle, β, is called the refractive index of themedium, x. Brix is related to the concentration of dissolved solids,e.g., sucrose, in a fluid and is related to the specific gravity of theliquid. Because the specific gravity of sucrose solutions is well known,it can also be measured by refractometers. Modern Brix meters aretypically digital refractometers that calculate the Brix value based ona refractive index. A Brix percentage scale is recommended by theInternational Committee of Uniform Method of Sugar Analysis (ICUMSA).Brix meters are used in the food industry for measuring the approximateamount of sugars in fruits, vegetables, juices, wine, soft drinks and inthe sugar manufacturing industry.

Electrical conductivity is used as a basis for assessing the progress ofelectrodialysis in the making of ascorbic acid as proposed in U.S. Pat.No. 5,702,579 by Veits. Conductivity is the ability of a material toconduct electric current. Instruments that measure conductivity mayemploy an anode and a cathode as two plates of a circuit having anelectrical potential applied across the plates, which may be via a sinewave voltage, and the current that passes through the solution andcompletes the circuit is measured. FIG. 2 is a depiction of an aspect ofan exemplary instrument for measuring conductivity employing an anodeand a cathode as two plates. The graphs shown in FIGS. 3A and 3Billustrate the relationship between conductivity and ion concentrationfor two common solutions. Notice that the graph is linear for a sodiumchloride solution, but nonlinear for highly concentrated sulfuric acid.Ionic interactions can alter the linear relationship betweenconductivity and concentration in some highly concentrated solutions.FIG. 3A is a graphical depiction of the relationship of the ionconcentration of sodium chloride in solution with electricalconductivity. FIG. 3B is a graphical depiction of the relationship ofthe ion concentration of sulfuric acid in solution with electricalconductivity. FIG. 4 is a graphical depiction of an in-line placement ofa circuit element of an electrical conductivity measuring device.

Most conductivity meters have a two-electrode cell, as in FIG. 2,available in either dip or flow-through styles. The electrode surface isusually platinum, titanium, gold-plated nickel, or graphite.Four-electrode cells use a reference voltage to compensate for anypolarization or fouling of the electrode plates. The reference voltageensures that measurements indicate actual conductivity independent ofelectrode condition, resulting in higher accuracy for measuring overwide ranges. Conductivity (G), is the inverse of resistivity (R), i.e.,G=1/R, and conductivity may be determined from the voltage and currentvalues according to Ohm's law (V=IR) where since the charge on the ionsin solution facilitates the conductance of electrical currents, theconductivity of a solution may be proportional to the solution's ionconcentration.

Conductivity may be measured via sensor elements directly immersed inthe solution as in FIG. 2. Alternatively, one or more of the sensorelements may be encased in protective material before immersion orplaced outside of the direct flow path. FIG. 4 is a graphical depictionof an in-line placement of a circuit element of an electricalconductivity measuring device. Toroidal coils, having an annularpresentation to a flow line, may be positioned or encased so as not tobe in contact with the solution. Toroidal coils may be either encased ina polymeric material or are external to a flow through cell. A toroidalconductivity measurement is made by passing an AC current through atoroidal drive coil, which induces a current in the electrolytesolution. This induced solution current, in turn, induces a current in asecond toroidal coil, called the pick-up toroid. The amount of currentinduced in the pick-up toroid is proportional to the solutionconductivity.

SUMMARY

Method and system embodiments of the present invention control theascorbate concentration in produce treatments and particularly areexemplified in fresh cut fruit and vegetable treatments via measuredrefractivity and/or electrical conductivity of, and/or calcium ionspresent in, the treatment solution. Method and system embodiments of thepresent invention control the ascorbate concentration in producetreatments and particularly are exemplified in fresh cut fruit andvegetable treatments via measured refractivity and/or electricalconductivity of the treatment solution and/or the presence of calciumions measured via a calcium ion-specific electrode. For example, amethod embodiment of the present invention includes a process forcontrolling ascorbate concentration of fresh produce treatmentcomprising, not necessarily in the following order, the steps of: (1)providing a fresh produce treatment comprised of water and ascorbate;(2) measuring via a refractive index sensor, a first ascorbateconcentration in a first sample comprised of the fresh producetreatment; (3) measuring via an electrical conductivity, a secondascorbate concentration in at least one of: the first sample comprisedof the fresh produce treatment and a second sample comprised of thefresh produce treatment; (4) determining via a calcium ion-specificelectrode, a third ascorbate concentration in at least one of: the firstsample comprised of the fresh produce treatment, the second samplecomprised of the fresh produce treatment, and a third sample comprisedof the fresh produce treatment; (5) generating a measured ascorbateconcentration based on the first measured ascorbate concentration, thesecond measured ascorbate concentration, and the third, i.e. thedetermined, ascorbate concentration; (6) comparing the generatedascorbate concentration with a control set point value to generate adifference value; and (7) if the difference value is above a threshold,then feeding ascorbate into the fresh produce treatment. In someembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in incremental amounts viaa pulsed valve. In other embodiments of the process, the step of feedingascorbate into the fresh produce treatment comprises feeding ascorbatein incremental amounts via a pulsed valve executing pulsesproportionally cycled based on the difference value. In still otherembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in via a pump havingpumping cycles proportionally based on the difference value.

An exemplary system for controlling ascorbate concentration in a freshproduce treatment may comprise: (a) the fresh produce treatmentcomprised of water, and ascorbate ions; (b) a prism sensor to measurerefractive index in the fresh produce treatment; (c) a circuit tomeasure electrical conductivity of the fresh produce treatment; (d) acalcium ion-specific electrode to measure calcium ions of the freshproduce treatment; and (e) a controller having a processor andaddressable memory and having a converter function adapted to read aconcentration output from the prism sensor, adapted to read an outputfrom an electrical conductivity circuit and adapted to read an outputfrom the calcium ion-specific electrode, and wherein the controller isadapted to transmit ascorbate feed commands to a feed pump in responseto the concentration difference. In some system embodiments, thecontroller may be further adapted to condition the concentrationdifference via a threshold hysteresis and a delay to generate the one ormore ascorbate feed commands. In other system embodiments, the feed pumpmay be further adapted to output incremental amounts from an ascorbatesource via a pulsed valve executing pulses proportionally cycled basedon the difference value. In still other system embodiments, the feedpump may be further adapted to output ascorbate from an ascorbate sourceby executing pumping cycles proportionally based on the differencevalue.

Method and system embodiments of the present invention control theascorbate concentration in produce treatments and particularly areexemplified in fresh cut fruit and vegetable treatments via measuredrefractivity and/or electrical conductivity of the treatment solution.For example, a method embodiment of the present invention includes aprocess for controlling ascorbate concentration of fresh producetreatment comprising, not necessarily in the following order, the stepsof: (1) providing a fresh produce treatment comprised of water andascorbate; (2) measuring via a refractive index sensor, a firstascorbate concentration in a first sample comprised of the fresh producetreatment; (3) measuring via an electrical conductivity, a secondascorbate concentration in at least one of: the first sample comprisedof the fresh produce treatment and a second sample comprised of thefresh produce treatment; (4) generating a measured ascorbateconcentration based on the first measured ascorbate concentration andthe second measured ascorbate concentration; (5) comparing the measuredascorbate concentration with a control set point value to generate adifference value; and (6) if the difference value is above a threshold,then feeding ascorbate into the fresh produce treatment. In someembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in incremental amounts viaa pulsed valve. In other embodiments of the process, the step of feedingascorbate into the fresh produce treatment comprises feeding ascorbatein incremental amounts via a pulsed valve executing pulsesproportionally cycled based on the difference value. In still otherembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in via a pump havingpumping cycles proportionally based on the difference value.

An exemplary system for controlling ascorbate concentration in a freshproduce treatment may comprise: (a) the fresh produce treatmentcomprised of water, and ascorbate ions; (b) a prism sensor to measurerefractive index in the fresh produce treatment; (c) a circuit tomeasure electrical conductivity of the fresh produce treatment; and (d)a controller having a processor and addressable memory and having aconverter function adapted to read a concentration output from the prismsensor and adapted to read an output from an electrical conductivitycircuit and wherein the controller is adapted to transmit ascorbate feedcommands to a feed pump in response to the concentration difference. Insome system embodiments, the controller may be further adapted tocondition the concentration difference via a threshold hysteresis and adelay to generate the one or more ascorbate feed commands. In othersystem embodiments, the feed pump may be further adapted to outputincremental amounts from an ascorbate source via a pulsed valveexecuting pulses proportionally cycled based on the difference value. Instill other system embodiments, the feed pump may be further adapted tooutput ascorbate from an ascorbate source by executing pumping cyclesproportionally based on the difference value.

Method and system embodiments of the present invention control theascorbate concentration in produce treatments and particularly areexemplified in fresh cut fruit and vegetable treatments via measuredcalcium ion concentrations. For example, a process for controllingascorbate concentration of fresh produce treatment may comprise: (1)providing a fresh produce treatment comprised of water and ascorbicacid; (2) measuring, via a refractometer, the Brix percentage of asample comprised of the fresh produce treatment; (3) comparing anascorbate concentration, derived from the measured Brix percentage, witha control set point value to generate a difference value; and (4) if thedifference value is above a threshold, then feeding ascorbate into thefresh produce treatment. In some embodiments of the process, the step offeeding ascorbate into the fresh produce treatment comprises feedingascorbate in incremental amounts via a pulsed valve. In otherembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in incremental amounts viaa pulsed valve executing pulses proportionally cycled based on thedifference value. In still other embodiments of the process, the step offeeding ascorbate into the fresh produce treatment comprises feedingascorbate in via a pump having pumping cycles proportionally based onthe difference value.

An exemplary system for controlling ascorbate concentration in a freshproduce treatment may comprise: (a) the fresh produce treatmentcomprised of water, calcium ions and ascorbic acid; (b) a refractometeradapted to sense Brix percentages in the fresh produce treatment; and(c) a controller having a processor and addressable memory and having acomparator adapted to difference ascorbate concentration derived fromsensed Brix percentages and a concentration set point wherein thecontroller is adapted to transmit one or more ascorbate feed commands toa feed pump in response to the concentration difference. In some systemembodiments, the controller may be further adapted to condition theconcentration difference via a threshold hysteresis and a delay togenerate the one or more ascorbate feed commands. In other systemembodiments, the feed pump may be further adapted to output incrementalamounts from an ascorbate source via a pulsed valve executing pulsesproportionally cycled based on the difference value. In still othersystem embodiments, the feed pump may be further adapted to outputascorbate from an ascorbate source by executing pumping cyclesproportionally based on the difference value.

Method and system embodiments of the present invention control theascorbate concentration in produce treatments and particularly areexemplified in fresh cut fruit and vegetable treatments via the measuredrefractivity of the treatment solution. For example, a process forcontrolling ascorbate concentration of fresh produce treatment maycomprise: (1) providing a fresh produce treatment comprised of water andascorbic acid; (2) measuring the electrical conductivity of a samplecomprised of the fresh produce treatment; (3) comparing an ascorbateconcentration, derived from the measured electrical conductivity, with acontrol set point value to generate a difference value; and (4) if thedifference value is above a threshold, then feeding ascorbate into thefresh produce treatment. In some embodiments of the process, the step offeeding ascorbate into the fresh produce treatment comprises feedingascorbate in incremental amounts via a pulsed valve. In otherembodiments of the process, the step of feeding ascorbate into the freshproduce treatment comprises feeding ascorbate in incremental amounts viaa pulsed valve executing pulses proportionally cycled based on thedifference value. In still other embodiments of the process, the step offeeding ascorbate into the fresh produce treatment comprises feedingascorbate in via a pump having pumping cycles proportionally based onthe difference value.

An exemplary system for controlling ascorbate concentration in a freshproduce treatment may comprise: (a) the fresh produce treatmentcomprised of water, calcium ions and ascorbic acid; (b) a subsystemcomprising a sensor adapted to measure the electrical conductivity ofthe fresh produce treatment and a converter adapted to derive ascorbateconcentration from measured electrical conductivity; and (c) acontroller having a processor and addressable memory and having acomparator adapted to difference the ascorbate concentration derivedfrom a sensed ascorbate concentration and a concentration set pointwherein the controller is adapted to transmit one or more ascorbate feedcommands to a feed pump in response to the concentration difference. Insome system embodiments, the controller may be further adapted tocondition the concentration difference via a threshold hysteresis and adelay to generate the one or more ascorbate feed commands. In othersystem embodiments, the feed pump may be further adapted to outputincremental amounts from an ascorbate source via a pulsed valveexecuting pulses proportionally cycled based on the difference value. Instill other system embodiments, the feed pump may be further adapted tooutput ascorbate from an ascorbate source by executing pumping cyclesproportionally based on the difference value.

Method and system embodiments of the present invention control theascorbate concentration in produce treatments and particularly areexemplified in fresh cut fruit and vegetable treatments via measuredcalcium ion concentrations. For example, a process for controllingcalcium ascorbate concentration of fresh produce treatment may comprise:(1) providing a fresh produce treatment comprised of water, calcium ionsand ascorbic acid; (2) measuring, via a calcium ion-specific sensor, acalcium ion concentration in a sample comprised of the fresh producetreatment; (3) comparing the measured calcium ion concentration with acontrol set point value to generate a difference value; and (4) if thedifference value is above a threshold, then feeding calcium ascorbateinto the fresh produce treatment. In some embodiments of the process,the step of feeding calcium ascorbate into the fresh produce treatmentcomprises feeding calcium ascorbate in incremental amounts via a pulsedvalve. In other embodiments of the process, the step of feeding calciumascorbate into the fresh produce treatment comprises feeding calciumascorbate in incremental amounts via a pulsed valve executing pulsesproportionally cycled based on the difference value. In still otherembodiments of the process, the step of feeding calcium ascorbate intothe fresh produce treatment comprises feeding calcium ascorbate in via apump having pumping cycles proportionally based on the difference value.

An exemplary system for controlling calcium ascorbate concentration in afresh produce treatment may comprise: (a) the fresh produce treatmentcomprised of water, calcium ions and ascorbic acid; (b) a calciumion-specific electrode adapted to sense calcium ion concentrations inthe fresh produce treatment; (c) a controller having a processor andaddressable memory and having a comparator adapted to difference asensed calcium ion concentration and a calcium ion concentration setpoint wherein the controller is adapted to transmit one or more calciumascorbate feed commands to a feed pump in response to the concentrationdifference. In some system embodiments, the controller may be furtheradapted to condition the concentration difference via a thresholdhysteresis and a delay to generate the one or more calcium ascorbatefeed commands. In other system embodiments, the feed pump may be furtheradapted to output incremental amounts from a calcium ascorbate sourcevia a pulsed valve executing pulses proportionally cycled based on thedifference value. In still other system embodiments, the feed pump maybe further adapted to output calcium ascorbate from a calcium ascorbatesource by executing pumping cycles proportionally based on thedifference value.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of exampleand not limitation in the figures of the accompanying drawings, and inwhich:

FIG. 1 is a graphic depiction of refraction;

FIG. 2 is a depiction of an aspect of an exemplary instrument formeasuring conductivity employing an anode and a cathode as two plates;

FIG. 3A is a graphical depiction of the relationship of the ionconcentration of sodium chloride in solution with electricalconductivity;

FIG. 3B is a graphical depiction of the relationship of the ionconcentration of sulfuric acid in solution with electrical conductivity;

FIG. 4 is a depiction of an in-line placement of circuit elements of anelectrical conductivity measuring device;

FIG. 5 is an exemplary top-level function lock diagram of a systemembodiment of the present invention;

FIG. 6 is an exemplary top-level function lock diagram of a systemembodiment of the present invention;

FIG. 7 is an exemplary top-level function lock diagram of a systemembodiment of the present invention;

FIG. 8 is an exemplary top-level function lock diagram of an estimatorof ascorbate concentration level in a sample based on both refractometryand measurements of electrical conductivity;

FIG. 9 is an exemplary top-level function lock diagram of a systemembodiment of the present invention;

FIG. 10 is an exemplary top-level function lock diagram of a systemembodiment of the present invention;

FIG. 11 is an exemplary top-level function lock diagram of a systemembodiment of the present invention; and

FIG. 12 is an exemplary top-level function lock diagram of an estimatorof ascorbate concentration level in a sample based on refractometry,measurements of electrical conductivity, and measurements from a calciumion-specific electrode.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The several embodiments of an ascorbate monitoring and control systemmeasure the refractive index as an indicator for the concentration ofascorbate in the fresh fruit or vegetable treatment which may becontained in a processed fresh produce treatment tank, a fresh produceprocessing tank or a processed produce dipping tank. A sensor subsystemis disclosed that accommodates continuous measurement and processconditions. The calibration relationship of refractive index toascorbate concentration is disclosed. The several embodiments disclose ameasurement analyzer that utilizes a linear calibration curve andexpresses a refractive index on a brix percentage scale. By having acontrol law and reference or set point, system embodiments provide foran automatic feed and replenishment of ascorbate concentrate on demand,that is, when the measured concentration deviates from the referencepoint. For diagnostic or other forensic purposes, measuredconcentrations of ascorbate and/or feed pump commands may be recordedand stored.

The ascorbate automated monitoring and control system for fresh cutfruits and vegetables may be specialized for monitoring and maintainingthe level of ascorbate in a solution via one or more means ofdetermining and controlling the level ascorbate as sampled from theprocessing, of dip, tank. An operating principle of control systemembodiments of the present invention includes in some embodiments thesensing of the refractive index of the process solution having avariation from pure or filtered water due in part to the control ofingredients in the solution, the measurement of refractivity may becorrelated to the concentration of ascorbate. Generally, the ascorbateestimation and control system may be described as the combination ofthree primary processes: (1) the measurement process; (2) the samplingprocess; and (3) the control and injection process.

FIG. 5 illustrates an exemplary system embodiment 100 having ameasurement analyzer 110 drawing water from both a water source 101 anda treatment tank 102 and a controller 130 outputting feed pump commands131 based on an output of a measurement analyzer 110 and a referencevalue 120. The sensor manifold includes a refractometer, such as a Brixdevice 112, and a converter 114 that takes the Brix percentage output as153 as its own input and in turn outputs 115 a derived ascobateconcentration. Responsive to the control law output 131, the feed pump108 dispenses ascorbate 140 into the treatment tank 102.

A first pump 108 may dispense ascorbate 140 to the treatment tank 102having water provided 160 from a water source 101.

A second pump 106 may be employed to sample and provide 152 solutionfrom the treatment tank to the measuring device 112 or an array ofmeasuring devices. After the sampled solution 154 has been subjected tothe one or more measuring devices, it may be returned to the treatmenttank 102 or conveyed to a waste water reservoir.

Measurement Process via Refractometry

The measurement analyzer of the present invention may be built aroundrefractometry and particularly may exploit a Brix meter. The refractiveindex is based upon the supposition that the refractive index in avacuum is one, i.e., the absolute refractive index, and, for aparticular medium passing light, the index varies with the wavelength oflight and temperature of the medium. A standard for reflectivity may berepresented by D where under the D-ray of natrium (r89 nm), thereflectivity index, N, of water at 20 degrees Celsius is 1.33299. TheBrix percentage scale is based on the refractive index of water(nD=1.33299) as the reference, i.e., at a Brix percentage of 0%. TheBrix percentage scale represents the weight of the sucrose expressed bypercentage, e.g., the sucrose weight in grams contained in 100 grams ofsucrose solution expressed as a percentage by weight. Therefore, thisscale corresponds with the sucrose concentration. However, while sugardissolves readily into water, most sample ingredients other than sugarare typically melted. The aggregate concentration of these ingredientsis correlated with the Brix percentage, so this makes the Brixpercentage scale a practical tool for measuring concentrations.Accordingly, any in-line, continuous monitoring, refractometer may beemployed to detect the refractive index of a sample and output the Brixpercentage value on a display. The refractive index of similarsubstances will vary with the temperature. When measuring the refractiveindex of a liquid by the refractometer, the measurement value carrieswith the sample temperature. By detecting the prism temperature, theindication value of the measurement is automatically corrected fortemperature by a built-in-processor so that the indicated value isidentical to the value measured at 20° C.

Sampling Pump

A pump is utilized to measure a representative sample of the processsolution which may be of a piston, peristaltic, centrifugal or othertype which may deliver a continuous sample.

Measurement Process via Electrical Conductivity

The basic unit of conductance is the siemen (S). Since cell geometryaffects conductivity values, standardized measurements are expressed inspecific conductivity units (S/cm) to compensate for variations inelectrode dimensions. Specific conductivity (C) is simply the product ofmeasured conductivity (G) and the electrode cell constant (L/A), where Lis the length of the column of liquid between the electrodes and A isthe area of the electrodes (FIG. 2).C=G×(L/A)Conductivity Meter Calibration and Cell Maintenance

Conductivity meters and cells should be calibrated to a standardsolution before being used. Select a standard that is closest to theconductivity of the solution to be measured. Polarized or fouledelectrodes must be replatinized or cleaned to renew active surface ofthe cell. In most situations, hot water with a mild liquid detergent isan effective cleanser.

The conductivity of some common solutions is shown in Table 1 below.

TABLE 1 Solution Conductivity Pure water 0.055 μS/cm Power plant boilerwater 1.0 μS/cm Good city water 50 μS/cm Ocean water 53 mS/cm 31.0% HNO3865 mS/cmConductivity Temperature Compensation

Conductivity measurements are temperature dependent. The degree to whichtemperature affects conductivity varies from solution to solution andcan be calculated using the following formula:G _(t) =G _(tcal){1+α(t−t _(cal))}where:G_(t)=conductivity at any temperature t in ° C.G_(tcal)=conductivity at calibration temperature t_(cal) in ° C.α=temperature coefficient of solution at t_(cal) in ° C.

Common alphas (a) are listed in Table 2 below. To determine the α ofother solutions, one may measure conductivity at a range of temperaturesand graph the change in conductivity versus the change in temperatureand then divide the slope of the graph by G_(tcal) to get α.

TABLE 2 Substance at 25° C. Concentration Alpha (α) HCl 10 wt % 1.56 KCl10 wt % 1.88 H₂SO₄ 50 wt % 1.93 NaCl 10 wt % 2.14 HF 1.5 wt % 7.20 HNO₃31 wt % 31.0

Exemplary meters may be either fixed or adjustable automatic temperaturecompensation referenced to a standard temperature—usually 25° C. Mostmeters with fixed temperature compensation use an α of 2% per ° C. (theapproximate α of NaCl solutions at 25° C.). Meters with adjustabletemperature compensation let you adjust the alpha factor to more closelymatch the alpha factor of your solution.

Conductivity Cells: Most conductivity meters have a two-electrode cell(FIGS. 2 and 4) available in either dip or flow-through styles. Theelectrode surface is usually platinum, titanium, gold-plated nickel, orgraphite. Four-electrode cells use a reference voltage to compensate forany polarization or fouling of the electrode plates. The referencevoltage ensures that measurements indicate actual conductivityindependent of electrode condition, resulting in higher accuracy formeasuring over wide ranges

The measurement of the ions may be made either by a contacting ornon-contacting conductivity sensor that is calibrated to the appropriaterange of concentration, and interfaces with a conductivity instrument inorder to display, via a calibrated scale, the measured value as apercentage of ascorbate. That is the conductivity instrument displaysthe value of ascorbate as a percentage concentration.

Sensor: The exemplary conductivity sensor is a non-contacting torroidalelectrode that is specially constructed with coils encapsulated in a PVChousing so that coating sucrose and pectin byproducts of the process maynot coat and affect the measurement accuracy. The non-contacting sensorcreates an electric current and as the process solution passes throughthe sensor, a change in the strength of the current generated isproportional to the ion concentration. The encapsulated design remainsprotected and thereby minimizing maintenance downtime.

The sensor design selected for the trials had a non-contactingconductivity sensor that is not susceptible to clogging. The sensingelement was encapsulated in a PVC housing and is protected from foulingfailure. Accordingly, the sensor measures the solution conductivity andthe ionic concentration, and the signal was then processed by aconductivity analytical process instrument.

Measurement/Sensor Manifold

A sensor manifold may be embodied to accept a continuous uninterruptedflow such as with a centrifugal or peristaltic pump, and due to themagnetic field generated by the sensor due to the two coils placedperpendicular to each other encapsulated in the PVC housing, a minimumtwo inch PVC Tee may be required to allow the field to remain stable forelectrical conductivity assessment. The liquid stream is fed into themanifold from the ascorbate process dip tank through the bottom, passesthrough the sensor housing, and exits from the side of the Tee housing.The stream exits the electrode housing and may be returned back to thedip tank.

Sampling Pump

The single peristaltic pump or a centrifugal magnetic drive seal-lesspump may be selected for some embodiments. The tubing of the peristalticpump for suction and discharge for the trials was polyethyleneone-quarter inch outer diameter (OD) and holds an approximate volume of2.5 milliliter per foot (ml/ft) of tubing, and for the centrifugal pump,it is a one-half inch reinforced hose. The sampling pump selectedpreferably delivers the sample from the dip tank to the sensor in lessthan two minutes.

Measurement Based on Refractometry

Operation of the sensor portion of the exemplary system may include thefollowing: (1) when power is supplied to the refractive index sensor,the Brix percentage measurement starts; (2) when the sample runs intothe sample inlet unit and the prism surface is filled with the solution,the Brix percentage value may be displayed on a screen; (3) when theprism surface contacts air, it may signal such contact via a displaysuch as the characters “LL.L”; (4) the instrument may then detect thetemperature of the prism unit, and the Brix percentage indication valuesare automatically corrected for temperature to indicate values identicalto the values measured at 20° C. when the sample temperature is in therange of 5 to 100° C.; (5) the temperature correction values for sucroseare selected due to the close refractive index of ascorbate and sucrose.

Measurement Based on Electrical Conductivity

A conductivity instrument is selected to process the signal from thesensor and displays the concentration of the ions, measured on a linearscale with the ability to enter a zero point and a reference pointcalibration. An electrode measurement system may be comprised of acontacting or non-contacting conductivity sensor which may be housed inan inline sensor manifold. An exemplary electrode-less toroidalconductivity sensor body containing a torroidal coil may be installed ina two inch PVC Tee.

When adjusting to the reference, confirm that the prism surface isclean. Before adjusting to the reference with distilled water, set thetemperature correction factor to “1.00.” Prudent steps in adjusting thereference include: (1) confirming that the sample inlet unit is properlyconnected to the piping; (2) running the distilled water or referencesample into the piping; (3) when the current Brix percentage isdisplayed, it may blink to allow for calibrations and during theblinking phase, incrementing and decrementing keys may be used to adjustthe Brix percentage to be displayed as 0.0% for distilled water or to aknown true value for a reference sample.

Ascorbate Concentration Control:

A proportional signal on a 4-20 milliamp (mA) scale may be processed bya programmable logic controller (PLC) or another control processor,which is programmed to display a concentration of ascorbate as apercentage of ascorbate. A control set point may be entered and therelay output from the controller will output 110VAC on/off or pulseproportional control output below the set point to dispense a liquidstock concentrate of ascorbate. Alternatively a proportional analogsignal output of a 4-20 mA from the controller may be used to determinethe rate at which the ascorbate powder will be dropped into the flume.

In exemplary configurations, there are three control laws by which anexternal dispensing device may be controlled by the instrument controlpanel: (1) ON/OFF control; (2) Pulse proportional control; and (3)Analog 4-20 mA control.

Exemplary ON/OFF Control:

FIG. 6 illustrates a system embodiment 200 having a pump 220, responsiveto an effector signal 203 based on a control law 210 for feedingascorbate 201 to a treatment tank 202 where the control law may have anON/OFF relay rated at 5 amps/120 VAC that may be used to turn on and offan external pump 220, valve, feeder or any other 115 VAC poweredexternal device. The feed rate of the pump 220 may not be modulated andas such, dispenses according to the pump 220 settings. The control law210 may be embodied as one or more instructions of a generalmicrocontroller, functioning as an ion concentration controller that mayhave a central processing unit (CPU) and addressable memory or may beembodied via a programmable logic controller (PLC). The control law inthe example of FIG. 6 differences 211, from the control set point value212, the ascorbate concentration 231 derived from the measuring device230, such a Brix device if refractometric, or circuitry for measuringelectrical conductivity, which is shown sampling a tapped flow 204 fromthe dip tank for an ascorbate concentration via a pump 205 pumping asample flow from the dip tank sample 202. An error signal 213, E,resulting from the differencing 211 may be delayed via a time delay 214of one or more sample cycles in order to accommodate the mixing timeconstant of the dip tank 202 as it receives ascorbate 201 via the pump220. A threshold hysteresis 215 is shown where an effector signaloutputs an “ON” value when the measured concentration falls below a setamount from the reference value.

An exemplary control set point may be 5.5% where the measurement scaleis selected at 0 to 10%, and an adjustable hysteresis dead band set at0.1% and a delay timer set to 5 seconds for the ON/OFF state change forthe relay. When the reading falls below the set point 5.5% by thehysteresis value 0.1, i.e. at a concentration reading of 5.4%, the relaywill turn ON after 5 seconds, turn ON the feed pump to add more calciumascorbate and raise the concentration until the value reaches 5.5%, andthen it will turn OFF. The change in concentration percentage value isslow and can take several minutes to change by a 0.1% concentration.Therefore the hysteresis and the delay timer and/or the inherent processtime delay work to prevent the relay from chattering between the ON andOFF state. The on-off embodiment may be applied when the calciumascorbate is in liquid form.

Exemplary Pulse Proportional Control:

The control relay described above may also be assigned as a pulseproportional relay. In this mode, the relay turns ON and OFF at a dutycycle frequency proportional to the extent to which the measured valuedeviates from the set point. The further the measured value isdetermined to be from the set point, the more frequently the relay willpulse, i.e., increase in the frequency of the duty cycle, andaccordingly the pump 220 will inject the solution at a higher rate. Thecloser to the set point the measured value is determined to be, theslower the rate of pulsing, and the decreasing pulse rate decreases topracticably zero when the set point is reached by the determined measurevalue. The rate, i.e., cycles per second, of pulsing may be selecteddepending on the capability of the receiving pump or valve. For example,the range assigned to the pulse band is equivalent to the selected rangeof the instrument, i.e., 0 to 10%, but from the set point value to thepoint of maximum pulse rate value. For example, if the set point is5.5%, the pulse output will begin as the reading falls below 5.5%, andreaches its maximum pulse rate at 0%.

Exemplary Analog 4-20 mA Output:

FIG. 7 illustrates a system 300 having an analog output 303 that may beused to proportionally actuate a feed valve 320 or pump in proportion tothe measured value 231. The control law 310 may be embodied in an ionconcentration controller that is a microcomputer having addressablememory and adapted to execute one or more machine readable instructionsto effect at least a proportional control or may be implemented indiscrete steps via a PLC. An error signal 313, E, may be generated bydifferencing 311 the measured calcium ion concentration 231 from thecontrol set point 312 and may be amplified into an effector signal 303by applying a proportional gain 314, κ. The proportional gain may alsohave output signal limiting as shown by example in the gain-limiterblock 315 of the control law 310. The feed pump 320 of FIG. 7 is shownto operate at a maximal frequency when the effector signal 303 is at itslowest value and to operate at a minimal frequency when the effectorsignal 303 is at its highest value. The analog signal embodiment may beapplied when the calcium ascorbate source is in powder form.

Details of a preferred exemplary embodiment are as follows: a scale of 0to 10% may be assigned equivalent to 4-20 mA output value, i.e., 4 mAequals 0% and 20 mA equals 10%. Differencing 4 mA from 20 mA yields 16mA and provides for an exemplary definitional point of 10% which in turnallows for an exemplary slope of 1.6 mA per percentage point which setsthe proportional gain. Accordingly in this example, an 8 mA range isallocated to accommodate a 5% concentration range. Since 4 mA is thebeginning of the mA scale, the full range ends with at least 12 mA.

This output signal varies accordingly with changing values and may beused to proportion the feeder. For example, a set point value of 5.5% is5.5×1.6+4=12.8 mA. Set the control pump input value of 0 cycles/min=to12.8 mA, and 4 mA to max strokes/min. Thus, the further (lower) themeasured value is from the set point, the faster the output pump willrun, or the greater the output valve will open, thereby creating aproportional feed system.

Dispensing Pump:

While ascobate may be dispensed in a solid, e.g., sodium ascorbate orcalcium ascorbate, or a liquid form as ascorbic acid in solution, thepreferred dispensing of ascorbate is in a liquid form of 25%weight-per-volume (w/v), however other concentrations may be used. Sincethe exemplary control system is configured to dispense on demand, thehigher the concentration, the less the dispensed volume is required toachieve the desired concentration in the dip tank. A peristaltic pumpwith variable speed adjustment may be selected at 85 gallons per day(GPD) capacity to allow for dip tank capacity variations.

Data Recording:

A data logger may be integrated into the instrument to interface with ascalable 4-20 mA output range signal proportional to a range of 1% to10% of an ascorbate concentration.

Calibration for Refractometry

Calibrating the exemplary system for practicable operation may bedescribed as calibrating within the sensor suite of instruments andcalibrating the effector, particularly the ascorbate dispensingmechanism. System calibration may be effected via an exemplaryadjustment to the reference. When adjusting to the reference, confirmthat the prism surface is clean. Before adjusting to the reference withdistilled water, set the temperature correction factor to “1.00.” Thefollowing steps may be employed for calibrating the system for aparticular Brix device: (1) confirming that the sample inlet unit isconnected to the system piping; (2) running distilled water or areference sample into the piping; (3) supplying power to the measurementand control system; and (4) placing the Brix device into a calibrationmode.

Calibration for Electrical Conductivity

Exemplary Sensor Check and Calibration: An exemplary calibrationprocedure that requires two solutions may include the steps of: (1)Preparing an, at least 200 ml, apple wash water sample by cutting piecesof peeled apples in a container, adding fresh tap water, and allowingthe admixture to soak, or sit, and mix for approximately one hour; (2)drawing, as zero calibration solution, a minimum of 200 ml as a sampleof apple process wash water prepared in step (1) that, at this pointshould not contain any ascorbate anti-oxidant; (3) measuring the levelof conductivity as a percentage value in the solution in order to use itas a zero calibration standard; (4) placing the sensor in-line andstarting the running process solution from the treatment tank thatcontains ascorbate; (5) determining the ascorbate level in the processsolution by titration and using the percentage value as the secondstandard; and (6) calibrating the second calibration standard as thereference point for the known value standard.

Ascorbate Measurements Based on Both Refractometry and ElectricalConductivity

In the ascorbate treatment process, certain errors may arise due to theprogressive increase in sucrose and fructose levels in the water bath.These may also vary due to the type of commodity and variety of productsthat are processed. In order to automatically compensate for theseprogressive variations, a combination method of both conductivity andrefractive index may be utilized as analytical techniques.

Refractive index measurements rely upon the change in the incident lightin order to quantify the concentration of dissolved solids. Sugarvariations follow a different slope of the relational line thanascorbate, and the combination of both follow a yet different slope. Asthe combined concentrations vary, the refractive index method is unableto clearly distinguish which of the two is causing the change. Thislimitation of the refractive index technique requires a correctionmethod to clearly define the contribution of ascorbate alone.

The conductivity of an ascorbate anion is directly proportional toconductivity, and is a clear and distinct method of determiningascorbate concentration. The contribution of background ions in the makeup water is negligible, and well below the margin of error. Sugars ingeneral do not contribute to conductivity measurements since they do notcontain free ionic charges. However conductivity alone is aninsufficient measurement since it does not have measurement sensitivityin high ranges as compared to a refractive index, and has insufficientresolution to detect minor changes in ascorbate concentration.

A combination of both refractive index and conductivity may be appliedto achieve both accuracy and sensitivity. The total refractive index isexpressed as a percentage of total dissolved solids, i.e., sugars plusascorbate on a slope calibrated to a combination of the two solutions.The total conductivity expressed as a percentage of ascorbateconcentration is subtracted from the percentage of the refractive indexon a brix scale, and the difference in measurement may be corrected bysubtracting it from the refractive index brix percentage and expressingthe corrected ascorbate measurement.

This adjusted measurement may then be used for a more accurate controlmechanism. FIG. 8 illustrates a portion 400 of a system where atreatment solution 401 is tapped 402 and sampled by a circuit 410 thatmeasures the electrical conductivity of the sample. The sample 403 maybe returned to the treatment solution or sent to a waste waterreservoir. In an alternative embodiment a portion 404 of the circuit 410may be disposed about a conduit carrying the treatment solution and beplaced in communication 405 with the remainder of the circuit measuringelectrical conductivity or the portion 403 may be immersed in thetreatment solution itself. The portion 400 of the system shown in FIG. 8also illustrates the treatment solution 401 tapped 406 and sampled by arefractometer 420 or other prism-based device that derivesconcentrations in solutions based on the refractive index. The sample407 may be returned to the treatment solution or sent to a waste waterreservoir. The electrical conductivity may also have a conversion modulethat converts output voltages to representative percentageconcentrations of electrolytes.

If the treatment solution is known via testing to contain electrolytesother than ascorbate and a corrected percentage concentration 414 isdesired for control feedback, an optional electrolyte concentration biasmay be established as an a priori bias 413 and combined 412 with theoutput 411 of the electrical conductivity measuring circuit 410. If thetreatment solution is known via testing to contain solute, other thanascorbate, such as sucrose and/or fructose, and a corrected percentageconcentration 424 is desired for control feedback, an optionalrefractometry-based concentration bias may be established as an a prioribias 423 and combined 422 with the output 421 of the refractometer 420.FIG. 8 illustrates an exemplary embodiment where the correctedpercentage concentration 414 output from the electrical conductivity maybe conditioned further, for example, by an electronic filter 430 such asa low-pass filter, which may reduce the higher frequency noise contentin the signal 414 to one that may be used as a correcting bias 431.

The exemplary portion 400 of the system may combine 432 the unfilteredsignal 414 or the filtered signal 431 representing the measuredelectrolyte concentration reflecting the concentration of ascorbic acidwith the corrected percentage concentration 424 from the refractometryin order to generate a corrective bias value 433. If the corrective biasvalue 433 over time becomes larger that a threshold value that mayrepresent the uncertainty range of measurements based on electricalconductivity, a switch 440 may close and allow for the combining 434 ofthe estimated sugar concentration bias 433 with the corrected percentageconcentration 424 to generate a corrected ascorbate concentrationmeasurement 435. Other embodiments for the exemplary portion 400 of thesystem include the outputs of the two measurement subsystemselectronically weighted based on minimizing statistical variances andbias effects, and the weighted measurement combined to produce thecorrected ascorbate measurement concentration.

The calibration relationship of calcium ions to calcium ascorbateconcentration is disclosed by the exemplary embodiments below. Theexemplary system embodiments have an in-line dilution system to exploitthe measurement process within the sensor sensitivity and resolution.The several embodiments disclose a measurement analyzer that utilizes asemi-logarithmic calibration curve to adjust its output for subsequentaction by the control effector. By having a control law and reference,e.g., a set point, system embodiments provide for an automatic feed andreplenishment of calcium ascorbate concentrate “on demand,” that is,when the measured concentration deviates from the reference point. Fordiagnostic or other forensic purposes, measured concentrations ofcalcium ascorbate and/or feed pump commands may be recorded and stored.

Ascorbate Concentration Control via Calcium Ion-Specific MeasurementDevices

The operating principle of control system embodiments of the presentinvention is that by sensing the specific calcium (Ca++) ions in theprocess solution and the application of a control law to a calciumascorbate source, the concentrations of ascorbate can be controlledaccording to the teachings of the instant specification. The sensing ofcalcium ions for an ascorbate control law is made practicable because,where the sole source of calcium ascorbate is controlled, the calciumion concentration is directly proportional to the concentration ofcalcium ascorbate. So, in an exemplary application of the ascorbatecontrol system to fresh cut fruits and vegetable, the calcium ascorbateautomated monitoring and control system for fresh cut fruits andvegetables may be specialized for monitoring and maintaining the levelof calcium ascorbate in solution.

Generally, the ascorbate estimation and control system may be describedas the combination of three primary processes: (1) the measurementprocess; (2) the sampling process; and (3) the control and injectionprocess.

Measurement Process

The measurement of the calcium ions may be made by an ion-specificelectrode (ISE) that is calibrated to the appropriate range ofconcentration, and interfaces with an ion specific instrument in orderto display, via a semi-logarithmic scale, the measured value as apercentage of calcium ascorbate. That is, the ISE instrument providesand may display the value of calcium ascorbate as percentageconcentration.

Exemplary Sensor: The exemplary calcium ion specific sensor has anion-specific membrane with a specially formulated reference electrolytethat allows a specific measurement of the free calcium ions in solution.The exemplary ion specific membrane is sensitive and could be saturatedby the process solution concentration, interfering ion activity and highelectron density. In order for a concentration to be in the preferredsensitivity range of the exemplary sensor, one may embody an inlinedilution system in order to remain within a commercially practicablemeasurement resolution, as well as, to minimize the clogging rate of thereference junction of the sensor membrane, and thereby minimizingmaintenance downtime.

A dilution rate of the calcium ascorbate process solution was determinedin laboratory studies with numerous trials from sample solutionsprepared having sliced apple wash bath as a base solution with varyingamounts of calcium ascorbate. The sensor design selected for the trialshad a refillable reference electrolyte with an easily flushablereference junction to clear clogging. The sensing element was PVCmembrane specific and selective for Ca++ and may have been formattedeither integrally or in combination with the reference electrode or mayhave been embodied as a separate mono electrode. Accordingly, the sensormeasured the Ca++ ion concentration and the signal was then processed byan ion specific instrument.

FIG. 9 illustrates an exemplary system embodiment 900 having a sensormanifold 910 drawing water from both a water source 901 and a treatmenttank 902 and a controller 930 outputting feed pump commands 931 based onthe output of a calcium ion-specific electrode 914 and a reference value920. The sensor manifold includes the calcium ion-specific electrode andan in-line mixer 912 that combines water drawn from the water source viaa first pump 904 with water drawn from the treatment tank 902 via asecond pump 906. Responsive to the control law output 931, the feed pump908 dispenses calcium acscorbate 940 into the treatment tank 902 and thecalcium ascorbate may be dispensed in liquid or powder form.

Measurement/Sensor Manifold: The sensor manifold 910 may be embodied toaccept two liquid streams and may preferably have a continuousuninterrupted flow, such as with a peristaltic pump. The water sourcefor the calcium ascorbate process treatment tank, or dip tank, may alsobe the water source for the first liquid stream 951 into the manifold asshown in FIG. 9 as an optional flow path 960. A second liquid stream 952is shown feeding into the manifold from the calcium ascorbate processdip tank. The two streams are shown feeding into a common line and theconfluence is then passed through a static in-line mixer 912 in order toensure a homogeneous stream. The combined stream 953 is shown as thenpassing into an electrode flow cell chamber where the sensing electrodeis located so that it may sense the Ca++ concentration. The combinedstream 954 is shown exiting the electrode housing and may be returnedback to the dip tank or may be sent to waste for recycling outside ofthe estimation and control system environment.

A calcium electrode measurement system may be comprised of a referenceelectrode and a calcium Ion Specific Electrode (ISE). These twoelectrodes may be separated or may be integrated as a single combinationelectrode.

An exemplary calcium ISE includes an electrode body containing an ionexchanger in a sensing module that contains a liquid internal fillsolution in contact with a gelled organophilic membrane containing acalcium selective ion exchanger. An electrode potential develops acrossthe membrane when the membrane is in contact with a calcium solution.Measurement of this potential against a constant reference potentialwith a digital millivolt (mV) or ion specific meter depends on the levelof free calcium ions in solution, and corresponds to the measuredpotential as described by the Nernst equation:E=E ₀ +S log X;where:E=measured electrode potential;E₀=reference potential (a constant);S=electrode slope (˜25 mV/decade); andX=level of calcium ions in solution.The activity X represents the effective concentration of the ions insolution. Total calcium concentration, C_(t), includes a concentrationof free calcium ions, C_(f), plus a concentration of complex calciumions, C_(b). Since the calcium electrode only responds to the free ion,the free ion concentration, C_(f), may be represented as:C _(f) =C _(t) −C _(b).The activity, X, may be related to the free ion concentration, C_(f), bythe activity coefficient γ by:X=γC _(f).Activity coefficients vary, and can depend on the total ionic strength,I, where the total ionic strength may be defined as:I=½Σ_(x) C _(x) Z _(x) ²where:

C_(x)=even concentration of ion X;

Z_(x)=charge on ion X; and

E=sum of all types of ions in the solution.

In the case of high and constant ionic strength relative to the sensedion concentration, the activity coefficient, γ, is constant and theactivity, X, is directly proportional to the concentration.

Reference Electrode:

When two solutions of different compositions are brought in to contactwith one another, liquid junction potentials arise. Millivolt (mV)potentials occur from the inter-diffusion of ions into the solutions.Electrode charges will be carried unequally across the solution boundaryresulting in a potential difference between the two solutions, since theions diffuse at different rates. It is important that the potential bethe same when the reference is in the standardizing solution and thesample, otherwise a change in liquid junction potential will appear asan error in the measured electrode potential.

The composition of the liquid junction filling solution in the referenceelectrode affects the speed with which the positive and negative ions inthe filling solution diffuse in to the sample and should beequitransferent. No junction potential will result if the positive andnegative charge carried into the sample is equal.

The Sampling Process

Sampling Pump: The exemplary single peristaltic pump has a dual headwith independent flow rate adjustment. The peristaltic pumps may be adual head or two single head pumps and may also be fixed or variablespeed, for example those capable of delivering a 1:10 ratio of calciumascorbate to water. The tubing of the peristaltic pump for suction anddischarge for the trials is polyethylene one-quarter inch outer diameter(OD) and holds an approximate volume of 2.5 ml/ft of tubing. Thesampling pump selected preferably delivers the sample from the dip tankto the sensor in less than 20 minutes.

Measurement and Control

An ion specific measurement instrument 114 is shown having been selectedto process the signal from the sensor and displays the concentration ofthe specific ion, that in this example is measured on a semi-logarithmicscale separated by at least 14 mV to 25 mV per decade change inconcentration of the specific Ca++divalent cation. A control set pointas a reference 120 may be entered and the output 131 from the controllerin some embodiments will output a 110 VAC on/off or a pulse proportionalcontrol output below the set point in order to dispense, via the feedpump 108, a liquid stock concentrate of calcium ascorbate. Alternativelya proportional analog signal output of a 4-20 mA from the ionconcentration controller may be used to determine the effect rate, atthe feed pump 108, at which the calcium ascorbate powder will be droppedinto the flume.

The Control and Injection Process

Control: In an exemplary configuration, there are three control laws bywhich an external dispensing device may be controlled by the instrumentcontrol panel: (1) ON/OFF control; (2) Pulse proportional control; and(3) Analog 4-20 mA control.

Exemplary ON/OFF Control

FIG. 10 illustrates a system embodiment 1000 having a pump 1020,responsive to an effector signal 1003 based on a control law 1010 forfeeding calcium ascorbate 1001 to a treatment tank 1002 where thecontrol law may have an ON/OFF relay rated at 5 amps/120 VAC that may beused to turn on and off an external pump 1020, valve, feeder or anyother 115 VAC powered external device. The feed rate of the pump 1020may not be modulated and as such, dispenses according to the pump 1020settings. The control law 1010 may be embodied as one or moreinstructions of a general microcontroller, functioning as an ionconcentration controller, that may have a central processing unit (CPU)and addressable memory or may be embodied via a programmable logiccontroller (PLC). The control law in the example of FIG. 10 differences1011, from the control set point value 1012, the calcium ionconcentration 1031 derived from the calcium ion-specific electrode 1030which is shown sampling a confluence 1004 from the dip tank for acalcium ion concentration via a mixer 1005 that mixes the dip tanksample 1007,1009 with a water source 1008. An error signal 1013, E,resulting from the differencing 1011 may be delayed via time delay 1014of one or more sample cycles in order to accommodate the mixing timeconstant of the dip tank 1002 as it receives calcium ascorbate 1001 viathe pump 1020. A threshold hysteresis 1015 is shown where an effectorsignal outputs an “ON” value when the measured concentration falls belowa set amount from the reference value.

An exemplary control set point may be 5.5% where the measurement scaleis selected at 0 to 10%, and an adjustable hysteresis dead band set at0.1% and a delay timer set to 5 seconds for the ON/OFF state change forthe relay. When the reading falls below the set point 5.5% by thehysteresis value 0.1, i.e. at a concentration reading of 5.4%, the relaywill turn ON after 5 seconds, turn ON the feed pump to add more calciumascorbate and raise the concentration until the value reaches 5.5%, andthen it will turn OFF. The change in concentration percentage value isslow and can take several minutes to change by a 0.1% concentration.Therefore the hysteresis and the delay timer and/or the inherent processtime delay work to prevent the relay from chattering between the ON andOFF state. The on-off embodiment may be applied when the calciumascorbate is in liquid form.

Exemplary Pulse Proportional Control

The control relay described above may also be assigned as a pulseproportional relay. In this mode, the relay turns ON and OFF at a dutycycle frequency proportional to the extent to which the measured valuedeviates from the set point. The further the measured value isdetermined to be from the set point, the more frequently the relay willpulse, i.e., increase in the frequency of the duty cycle, andaccordingly the pump 1020 will inject the solution at a higher rate. Thecloser to the set point the measured value is determined to be, theslower the rate of pulsing, and the decreasing pulse rate decreases topracticably zero when the set point is reached by the determined measurevalue. The rate, i.e., cycles per second, of pulsing may be selecteddepending on the capability of the receiving pump or valve. For example,the range assigned to the pulse band is equivalent to the selected rangeof the instrument, i.e., 0 to 10%, but from the set point value to thepoint of maximum pulse rate value. For example, if the set point is5.5%, the pulse output will begin as the reading falls below 5.5%, andreaches its maximum pulse rate at 0%.

Exemplary Analog 4-20 mA Output:

FIG. 11 illustrates a system 1100 having an analog output 1103 that maybe used to proportionally actuate a feed valve 1120 or pump inproportion to the measured value 1031. The control law 1110 may beembodied in an ion concentration controller that is a microcomputerhaving addressable memory and adapted to execute one or more machinereadable instructions to effect at least a proportional control or maybe implemented in discrete steps via a PLC. An error signal 1113, ε, maybe generated by differencing 1111 the measured calcium ion concentration1031 from the control set point 1112 and may be amplified into aneffector signal 1103 by applying a proportional gain 1114, κ. Theproportional gain may also have output signal limiting as shown byexample in the gain-limiter block 1115 of the control law 1110. The feedpump 1120 of FIG. 11 is shown to operate at a maximal frequency when theeffector signal 1103 is at its lowest value and to operate at a minimalfrequency when the effector signal 1103 is at its highest value. Theanalog signal embodiment may be applied when the calcium ascorbatesource is in powder form.

Details of a preferred exemplary embodiment are as follows: a scale of 0to 10% may be assigned equivalent to a 4-20 mA output value, i.e., 4 mAequals 0% and 20 mA equals 10%. Differencing 4 mA from 20 mA yields 16mA and provides for an exemplary definitional point of 10% which in turnallows for an exemplary slope of 1.6 mA % which sets the proportionalgain. Accordingly in this example, an 8 mA range is allocated toaccommodate a 5% concentration range. Since 4 mA is the beginning of themA scale, the full range ends with at least 12 mA.

This output signal varies accordingly with changing values and may beused to proportion the feeder. For example, a set point value of 5.5% is5.5×1.6+4=12.8 mA. Set the control pump input value of 0 cycles/min=to12.8 mA, and 4 mA to max strokes/min. Thus, the further (lower) themeasured value is from the set point, the faster the output pump willrun, or the greater the output valve will open, thereby creating aproportional feed system.

Dispensing Pump:

While calcium ascorbate may be dispensed in a solid or a liquid form,the preferred dispensing of calcium ascorbate is in a liquid form of 50%w/v, however other concentrations may be used. Since the control systemis designed to dispense on demand, the higher the concentration, theless the dispensed volume is required to achieve the desiredconcentration in the dip tank. A peristaltic pump with variable speedadjustment may be selected at 85 GPD capacity to allow for dip tankcapacity variations.

Data Recording:

A data logger may be integrated into the instrument to interface with ascalable 4-20 mA output range signal proportional to a range of 1% to10% of a calcium ascorbate concentration.

Sensor Calibration

Calibrating the exemplary system for practicable operation may bedescribed as calibrating within the sensor suite of instruments andcalibrating the effector, particularly the calcium ascorbate dispensingmechanism.

An exemplary sensor check, or calibration, may be disclosed for appleprocessing as follows: (1) one may prepare a 100 ml solution of 1% and10% calcium ascorbate in apple process water: (a) in order to prepare a1% solution, take 100 ml of water in which sliced apples have beenwashed, and where the water does not contain any calcium ascorbate andthen add 1 gram of powder calcium ascorbate and stir until dissolved;(b) in order to prepare a 10% solution, take 100 ml of water in whichsliced apples have been washed, and where the water does not contain anycalcium ascorbate and add 10 grams of powder calcium ascorbate and stiruntil dissolved; (2) one may prepare a 100 ml solution of diluted 1:10of 1% and 10% calcium ascorbate in water that may be added to thecontents of the dip tank: (a) in order to prepare a 1:10 dilution of 1%solution, one may take 90 ml of water and 10 ml of the 1% solution(prepared in the preceding step of step (1)(a)), (b) in order to preparea 1:10 dilution of 10% solution, one may take 90 ml of water and 10 mlof the 10% solution (prepared in the preceding step of step (1)(b)); (3)one may then connect the calcium electrode to the ion specific meter andplace the meter into a 0.1 mV resolution mode; (4) one may measure the1% solution prepared in step (2)(a) and note the mV value and measurethe mV value of the 10% solution (prepared in the preceding step of step(2)(b)); and (5) one may note that the slope of the electrode is thevalue difference between the two samples separated by a one decadeconcentration, i.e., 1% and 10% and that the slope should be between 14and 25 mV.

Alternatively, a second exemplary sensor calibration method relying ontwo liquid solutions may be used for cut and peeled apple processing:(1) draw a minimum 200 ml sample of apple process wash water containingthe ascorbate anti-oxidant; (2) measure the level of ascorbate in thesolution by titration where the sample preferably has an ascorbate levelof at least 5% (and is typically a level that may be shared by thesecond step of the previous exemplary method of calibration); (3)prepare at least a 200 ml apple wash water sample by cutting pieces ofsome peeled apples in a container and adding fresh tap water and allowabout one hour for soaking and mixing; (4) draw 90 ml of the sampleprepared in step (3) and draw 10 ml from standard 2, i.e., the solutionprepared at step two of the preceding process, in order to preparestandard 1 at 1/10th the concentration of standard 2 (by noting the twovalues: if standard 2 is =5%, then standard 1 is 1/10th of standard 2and is =0.5%); and (5) one may then connect the calcium electrode to theion specific meter and place the meter into a 0.1 mV resolution mode;one may measure the standard no. 1 solution prepared in step 4 and notethe mV value and measure the mV value of the standard no. 2 solution(prepared in the preceding method's step (2)) one may note that theslope of the electrode is the value difference between the two samplesseparated by a one decade concentration, i.e., 0.5% and 5% and that theslope should be between 14 and 25 mV.

Control System Calibration

An exemplary control system calibration may be described as follows: (1)With the system in operational configuration with the manifold, andsampling pumps connected and electrode placed in the manifold perform aninline calibration; (2) Connect the suction tube of the sampling pump tothe 1% calcium ascorbate standard solution prepared in (1)(a) and runthe 1:10 inline water dilution pump. Wait until the sample reaches theelectrode and select the calibration mode on the instrument: (a) Selectthe two point calibration; (b) Select 10 decade value; (c) Value offirst calibration point is 1%; and (d) After reading is stabilized,accept the first measurement; (3) Move the suction tube of the samplingpump to the 10% calcium ascorbate standard solution prepared in (1)(b)and run the 1:10 inline water dilution pump. Wait until the samplereaches the electrode: (a) Value of second calibration point is 10%; and(b) After the reading is stabilized, accept the second measurement; and(4) Re-connect the suction tubing to the process tank.

Ascorbate Measurements Based on Refractometry, Electrical Conductivityand Calcium Ions

In the measurement of ascorbate solutions in process applications, theremay be three analytical techniques applied in order to determine thepercentage concentration of ascorbate ions. Complexities that arise in acontinuous process due to the dynamically changing nature of theascorbate dip solution include variations in sugar content of commoditytypes, product variety, and the formulation of the ascorbate additiveand its derivatives.

For processing using calcium ascorbate, deriving ascorbate concentrationpercentages indirectly based on a calcium ion-specific electrode is atechnique that, as described above, may be effective in correlating themeasured calcium ion concentration to the concentration of ascorbatepercentage. However, inaccuracies may arise if different formulationsand additives within the ascorbate are introduced that may be calcium orother interfering ion bases. For example, the calcium ascorbateformulation may at times also contain calcium carbonate, which can throwoff the correlation between calcium ions and ascorbate. Otherformulations of ascorbate, such as sodium ascorbate, may be used wherecalcium ions are not present at all. In such instances, the calcium ionspecific measurement technique is limited.

A combination of all three analytical techniques of deriving ascorbatepercentages: calcium ion-specific measurement, refractive index; andconductivity may be applied to achieve greater reliability, accuracy andsensitivity. The total refractive index is expressed as a percentage oftotal dissolved solids, i.e., sugars plus ascorbate on a slopecalibrated to a combination of the two solutions. Referring to FIG. 12,the total conductivity expressed as a percentage of ascorbateconcentration 1214 may be subtracted from the percentage of refractiveindex on a brix scale 424, and the difference in measurement may becorrected by subtracting 434 it from the refractive index brixpercentage 424 and outputting the corrected ascorbate measurement 1235.In the case of ascorbate solutions where calcium ascorbate derivative isutilized, the calcium ion-specific sensor 1030 (FIG. 10) may be used asa comparative reference to verify accuracy of the device measuringelectrical conductivity, and a correction factor may be programmed, as acalibrating value 1210, to adjust 1220 the slope of the conductivitymeasurement 414, which then accounts for all the ions and is correctedspecifically for calcium as the ion species of choice. Then thecomparative value of refractive index 424 is used corrected for by thecalcium adjusted conductivity measurement 1233 in order to provide thebest accuracy in the face of rising sugars and complex ioniccompositions.

This adjusted measurement 1235 may then be used for a more accuratecontrol mechanism. FIG. 12 illustrates a portion 1200 of a system wherea treatment solution 401 is tapped 402 and sampled by a circuit 410 thatmeasures the electrical conductivity of the sample. The sample 403 maybe returned to the treatment solution or sent to a waste waterreservoir. In an alternative embodiment a portion 404 of the circuit 410may be disposed about a conduit carrying the treatment solution and beplaced in communication 405 with the remainder of the circuit measuringelectrical conductivity or the portion 403 may be immersed in thetreatment solution itself. The portion 400 of the system shown in FIG.12 also illustrates the treatment solution 401 tapped 406 and sampled bya refractometer 420 or other prism-based device that derivesconcentrations in solutions based on the refractive index. The sample407 may be returned to the treatment solution or sent to a waste waterreservoir. The electrical conductivity may also have a conversion modulethat converts output voltages to representative percentageconcentrations of electrolytes.

If the treatment solution is known via testing to contain electrolytesother than ascorbate and a corrected percentage concentration 414 isdesired for control feedback, an optional electrolyte concentration biasmay be established as an a priori bias 413 and combined 412 with theoutput 411,416 of the electrical conductivity measuring circuit 410. Ifthe treatment solution is known via testing to contain solute, otherthan ascorbate, such as sucrose and/or fructose, and a correctedpercentage concentration 424 is desired for control feedback, anoptional refractometry-based concentration bias may be established as ana priori bias 423 and combined 422 with the output 421 of therefractometer 420. FIG. 12 also illustrates an exemplary embodimentwhere the corrected percentage concentration 1214 output from theelectrical conductivity may be conditioned further, for example, by anelectronic filter such as a low-pass filter, which may reduce the higherfrequency noise content in the signal 414 to one that may be used as acorrecting bias 1231.

The exemplary portion 1200 of the system may combine 432 the unfilteredsignal 414 or the filtered signal 1231 representing the measuredelectrolyte concentration reflecting the concentration of ascorbic acidwith the corrected percentage concentration 424 from the refractometryin order to generate a corrective bias value 1233. If the correctivebias value 1233 over time becomes larger that a threshold value that mayrepresent the uncertainty range of measurements based on electricalconductivity, a switch 440 may close and allow for the combining 434 ofthe estimated sugar concentration bias 1233 with the correctedpercentage concentration 424 to generate a corrected ascorbateconcentration measurement 1235. Other embodiments for the exemplaryportion 1200 of the system include the outputs of the three measurementsubsystems, i.e., calcium ion-specific electrode, electricalconductivity measuring device, and refractometry, electronicallyweighted based on minimizing statistical variances and bias effects, andthe weighted measurement combined to produce the corrected ascorbatemeasurement concentration.

Although this invention has been disclosed in the context of certainembodiments and examples, it will be understood by those of ordinaryskill in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. In addition, while a number of variations of the invention havebeen shown and described in detail, other modifications, which arewithin the scope of this invention, will be readily apparent to those ofordinary skill in the art based upon this disclosure. It is alsocontemplated that various combinations or subcombinations of thespecific features and aspects of the embodiments may be made and stillfall within the scope of the invention. The embodiments of the presentinvention may be embodied in a set of program instructions, e.g.,software, hardware, or both—i.e., firmware. Accordingly, it should beunderstood that various features and aspects of the disclosedembodiments can be combined with or substituted for one another in orderto form varying modes of the disclosed invention. Thus, it is intendedthat the scope of the present invention herein disclosed should not belimited by the particular disclosed embodiments described above.

What is claimed is:
 1. A system for controlling ascorbate concentrationin a fresh produce treatment comprising: the fresh produce treatmentcomprised of water, and ascorbate ions; a prism sensor to measurerefractive index in the fresh produce treatment; a circuit to measureelectrical conductivity of the fresh produce treatment; a calciumion-specific electrode; and a controller comprising a processor, aconverter, and a non-transitory readable medium having instructionstherein, wherein the converter: (1) reads a concentration output fromthe prism sensor, (2) reads an output from an electrical conductivitycircuit, and reads an output from a calcium specific electrode, andwherein the controller transmits ascorbate feed commands to a feed pumpin response to a determined concentration difference value between adetermined ascorbate concentration percentage and a target concentrationpercentage.
 2. The system of claim 1 wherein the controller conditionsthe concentration difference via a threshold hysteresis and a delay togenerate the one or more ascorbate feed commands.
 3. The system of claim1 wherein the feed pump outputs incremental amounts from an ascorbatesource via a pulsed valve executing pulses proportionally cycled basedon the difference value.
 4. The system of claim 1 wherein the feed pumpoutputs ascorbate from an ascorbate source by executing pumping cyclesproportionally based on the difference value.